158 Fertilisers [pt. hi 



In the United States considerable attention has been 

 devoted to the possibiHty of extracting potash from 



Tons per 

 acre 

 25 







No Super Super Super 



minerals only and potassium 



potassium and 

 salts magnesium 

 salts 



Fig. 33. Effect of mineral manures on the yield of mangolds already 



receiving nitrogenous manure (ammonium salts and rape cake). 



Barnfield, Rothamsted. (Roots, tons per acre, average 37 years, 



1876-1912.) 



feldspar especially when carried out as part of the pro- 

 cess for manufacturing Portland cement. 



